There's an Android app for Lemmy called jerboa which is still in it's early stages but quite usable. Federation to other instances is automatic except when an instance admin defederates to instances they think are problematic for the community or whatever. Don't quote me on that I'm still new here as well lol

If you want to subscribe to another instance community, you can either enter its full URL in the search (for instance beehaw.org/c/technology) or if someone else is already subscribed you can find it listed by going to the communities section here and selecting the “all” filter.